1. Field of the Invention
The present invention relates to a semiconductor process, and more particularly, to a method of reducing resistance in a conductor.
2. Description of the Prior Art
In a multilevel metallization process, metallic conductive layers connect each of the metal oxide semiconductor (MOS) transistors on the semiconductor wafer to form a stacked circuit structure. The multilevel metallization process improves the integration of the semiconductor wafer and is therefore commonly applied to very large scale integration (VLSI) processes.
FIG. 1 is a schematic diagram of a prior art conductor and plurality of contact plugs. In the prior art method, a sputtering method is used to deposit an Aluminum (Al) layer on the surface of a semiconductor wafer as the first step in the formation of an Aluminum-Copper/ Titanium/ Titanium nitride (Al/Ti/TiN) conductive layer. However, the result may cause the opening of the Al wire due to electron migration of the Al atoms. To prevent such a problem from occurring, the prior art method adds approximately 0.5%-4% Copper (Cu) to the Al to form an Alxe2x80x94Cu layer 10.
Secondly, a Ti metal layer 12 is deposited on the surface of the Alxe2x80x94Cu layer 10 with a thickness of deposition of approximately 200-500 angstroms. Then, a TiN layer is formed on the surface of the Ti layer 12 as a barrier layer 14 with a thickness of approximately 500-1500 angstroms. In general, there are two different methods for the formation of the barrier layer 14. One method is a proceeding nitridation of Ti layer 12 and the other method is the use of areactive sputtering method to directly deposit a TiN layer on the surface of Ti layer 12.
After formation of the conductive layer, its pattern is defined by both lithography and etching processes. Then, a dielectric layer is formed between the MOS transistor and each conductive layer to separate and protect the devices on the semiconductor wafer. In a multilevel metallization process, a contact plug is formed within the dielectric layer to act as a conductive wire between each MOS transistor and conductive layer to form a complete electrical device.
In the prior art method of fabricating a contact hole on the surface of a semiconductor transistor, a plurality of plug holes are formed within a dielectric layer 16. The plug holes lead to the surface of an outside-connecting conductive field, such as a source/drain of a MOS transistor or the surface of other lower conductors. Then, a glue layer and a Tungsten (W) layer are deposited on both the dielectric layer 16 and on the conductive field on the bottom of each plug hole. Next, an etch back process is performed to align the surface of the W layer with the surface of the dielectric layer 16 to form contact plugs 18.
In the prior art method, a sputtering temperature greater than 400xc2x0 C. is required to deposit the W layer. However, the increase in heat affects the structure of the conductive layer. Under the temperature of W formation, Ti is very easily reactive with Al. It leads to the formation of a high resistance Alxe2x80x94Ti complex which increases the resistance of the conductor to affect the electrical performance of the conductor.
It is therefore a primary objective of the present invention to provide a method of reducing resistance in an Al-containing conductor to solve the mentioned above problem.
In the present invention an Al oxide layer is first formed on the surface of the Al-containing conductor followed by the formation of a Ti layer and a barrier layer above the Al oxide layer, respectively. Finally, a W contact plug is formed above the barrier layer. The Al oxide layer functions in preventing a high temperature reaction between the Ti layer and the conductor during formation of the W contact plug, to avoid the influence of resistance in the Al-containing conductor.
In the present invention, a thin Al oxide layer is formed on the surface of the conductor to effectively prevent a high temperature reaction between the Ti layer and the Al layer during formation of the W plug and prevent an increase of resistance in the conductor.